Prior art FIG. 1 illustrates a two-wheel drive motorcycle 10 of the type manufactured by the assignee of the present application. The motorcycle 10 is designed to be particularly adept at maneuvering over rough terrain at relatively low speeds, e.g., up to about 35 to 40 mph. For example, the front wheel drive feature facilitates climbing over steep, rough terrain. The general operation of the prior art motorcycle 10 shown in FIG. 1 is described in U.S. Pat. No. 4,702,341 and U.S. patent application Ser. No. 11/503,587, both of which are assigned to the assignee of the present application and are incorporated herein by reference. Briefly, in reference to prior art FIG. 1, the motorcycle 10 has an engine 12, mounted to the frame 14, that provides power to a transmission 16 via a belt drive torque converter. The transmission 16 has a rear output shaft 18 to which is connected a sprocket for a chain 20 that drives a sprocket 24 secured to the rear wheel 22. The sprocket 24 on the rear wheel 22 preferably has 60 teeth. The transmission 16 also has another output shaft facing forward (not shown in FIG. 1) which provides power through a one-way overrunning clutch mechanism 26, a universal joint 28, and miter gears 30 to a front wheel chain drive 32. Chain 32 drives a sprocket 34 secured to the front wheel 36. The sprocket 34 on the front wheel 36 also preferably has 60 teeth. For purposes of illustration, FIG. 1 shows the sprocket 24 on the rear wheel 22 and the sprocket 34 on the front wheel 36 on the same side of the motorcycle 10. In practice, the sprocket 24 for the rear wheel 22 and sprocket 34 for the front wheel are located on opposite sides of the motorcycle 10 in order to balance torque. The power ratio between the rear wheel 22 and the front wheel 36 is preferably maintained at a ratio of about 1:1, with the overrunning clutch 26 providing slip for the front wheel drive and allowing the front wheel to turn faster than the rear as required when the motorcycle is making a turn.
As mentioned, the type of motorcycle 10 shown in FIG. 1 is typically used off road often to contemplate rough or steep terrain, or to haul heavy objects. The seat 38 on motorcycle 10 is typically mounted to the frame 14 using a spring loaded shock absorber, which is not shown in the FIG. 1. The shock absorber provides a relatively smooth ride to the user even when the rear wheel 24 encounters rough terrain. However, two-wheel drive motorcycles of the type shown in FIG. 1 do not incorporate the ability to absorb shock impacting the front wheel. The difficulty of absorbing shocks impacting the front wheel 36 resides mostly in the fact that the sprocket-to-sprocket distance for the front wheel chain drive 32 must be maintained during the course of operation to ensure proper chain tension. Providing shock absorbers within the front steering fork 40 is impractical because it would necessarily require the sprocket-to-sprocket distance for the front wheel chain drive 32 to vary.
Sudden impact on the front wheel 36 even at moderate speeds can produce normal forces many times greater than the forces predicted for typical weights and traction. At these instants, the rider may lose traction of the front wheel 36. Often, users will use low pressure tires, or under inflate tires to say 3 to 5 psi to improve traction when rough terrain is expected.
Also, repeated impact of the front wheel can lead to premature failure of the front wheel chain drive. Designing a front wheel chain drive with sufficient strength to handle large and unpredictable shock forces and torques is difficult.